This invention relates to a plug-type coke oven door for venting coke oven gases through a central vertical hole open at the top and bottom of the plug to relieve the pressure of gases generated at the bottom of a coal charge in a coke oven chamber. More particularly, the present invention relates to such a plug-type coke oven door embodying a particular construction and arrangement of parts to avoid the development of destructive thermal stresses in the plug due to rapid heating and cooling.
Environmental emissions are a problem by today's standards that must be overcome when operating a battery of horizontal recovery-type coke oven chambers. One principal form of pollution occurs from coke oven chambers when smoke and hydrocarbons escape from coke oven doors at both ends of an oven chamber. The integrity of seals for coke oven doors is difficult to maintain throughout long periods of operation. The doors must be removed at the conclusion of the coking process for pushing operations. During the early part of the coking cycle, large volumes of coke oven gas evolve. While a collector main is used to withdraw the gas, these collector mains communicate only with the free space above a coal charge and the gas must somehow reach this free space for withdrawal by the collector main. The flow paths for the gas through a coal charge from the bottom of a coke oven chamber are generally upward along courses of least resistance.
Coke oven gas generated at the middle of a coal charge in the coke oven chamber will flow generally upwardly through the coal charge to the free space thereabove and then through the ascension pipe into the collector main. On the other hand, a quantity of gas generated near the end of the coke oven chamber is more likely to travel horizontally to a "gas channel" at the site of the coke oven door and then upwardly to the free space above the coal charge. The "gas channel" is a clearance or space between the coke oven door and the jamb. The gas channel runs along the entire circumference of the door so that, in theory, the gas channel provides a flow path for gas to travel from the bottom of the coke oven chamber to the free space above the coal charge. In this way, the pressure of the coke oven gas is relieved at the bottom of the coke oven door. In practice, however, the gas channel is anything but a clear space and is often blocked with coal, coke and/or tar. Peak gas pressures at the bottom of the coke oven door are generally reported at about 200 millimeters of water, although some reports describe peak gas pressures at over 700 millimeters of water. The gas pressure drops from its maximum to a few millimeters of water after several hours and settles down to an even lower pressure, usually around 2 millimeters of water, throughout the remaining part of the coking cycle. The pressure of coke oven gas in the gas channel must be contained by the usual door seal strip to avoid emissions from the coke oven door. By minimizing the pressure of coke oven gas in the gas channel, and thus the gas pressure against the door seal, emissions from the coke oven doors are correspondingly minimized.
The pressure of coke oven gas in the gas channel is dependent, inter alia, upon the gas pressure at the free space above a coal charge and any pressure drop occurring along the gas channel. Gas pressures in the free space above a coal charge are slightly greater than the collector main gas pressure. However, significantly higher gas pressures develop when restrictions in the free space occur due to carbon buildup or improper leveling of a coal charge as well as inadequate cleaning in the gas-conducting conduits of the ascension pipe network. For example, it has been observed that cleaning an overly dirty passage from the free space above the coal to the collector main caused the pressure in the gas channel to drop immediately from 46 millimeters of water to 6 millimeters of water and that heavy emissions from the door seal were immediately eliminated. While the occurrence of unusual operational conditions, such as the above, in a coke oven battery brings about undesirable emissions, the reliance on the gas channel to provide a flow space for coke gas from the bottom of the coke oven chamber to the top portion thereof is undesirable even under normal operating conditions because it imposes excessive demands upon the integrity of the door seal strip.
Techniques developed in the past to reduce the gas channel pressure include the formation of a coal seal by reducing the clearance between a refractory door plug and the wall of the coke oven chamber to prevent coal from entering the gas channel area. However, difficulties are frequently encountered when attempting to set the door in place when the gap between the plug and the oven wall is sufficiently reduced. A further technique to minimize restrictions to the flow of coke oven gas in the gas channel employs the concept of undercutting the plug on the coke oven door to enlarge the gas channel. However, due to the flow of hot gases in the enlarged gas channel, overheating of the coke oven door will occur. A still further technique is to provide an alternate flow path for the coke gas within an internal vertical hole along the entire length of the door plug to relieve pressures of coke oven gas at the bottom of the door. Side vents along the height of the plug leading into such a hole have also been used to relieve the pressure of coke oven gas at intermediate points along the height of the door. The concept of a vented plug is particularly desirable since the flow of pressurized coke gas is diverted from the gas channel area, thus relaxing demands on the integrity of the door seal strip. However, known vented plug designs suffer from acute disadvantages which the present invention is designed to overcome. A relatively small gas-conducting opening in some vented plug designs becomes plugged due to the buildup of tar and other material from the gases. This accumulation of foreign material can be removed through systematic cleaning operations, but this is not seen as an acceptable solution to the problem. An alternative suggestion in the past has been to provide a very large central opening in the plug surrounded by relatively thin plug walls so that the plug operates at a relatively high temperature and thereby burn deposits on the wall forming the gas-conducting channel in the plug. Because the wall thickness of the plug is small, large hoop stresses develop whereby thermal cracking is a serious problem that reduces the life of the door plug.